Fast switching high-voltage power devices such as IGBTs (Insulated Gate Bipolar Transistors) are used for controlling inductive loads in converters for variable speed drives. The inductive loads are, for example, electric motors. Such power devices are designed, depending on the intended use, to block some 100 V up to 6.5 kV. The converters include bridge circuits formed by power devices which are alternatingly switched on and off to generate an output voltage signal having a desired frequency. This is also known as pulse-width modulation (PWM). Multiple power devices can be combined, together with their free-wheeling diodes, in a module for high-voltage and high-current applications.
The power devices may generate heat during operation caused by switching losses and overload situations. For standard operation, the generated heat can be dissipated by a heat sink which is thermally coupled with the power devices. On the other hand, a temperature detection is often desired to monitor the temperature of the power devices and to ensure that the devices do not overheat. A temperature sensor is thus desired.
Conventionally, so-called PTC- or NTC-resistors have been integrated into power devices. PTC-resistors (positive temperature coefficient resistor) are resistors having a resistance which increases with the temperature. Different thereto are NTC-resistors (negative temperature coefficient resistor), the resistance of which decreases with temperature. Each type of resistors is defined by its specific temperature coefficient TC which is a measure to what extent the resistance changes with the temperature. A linear relation between resistance and temperature is desirable.
Typical PTC-resistors, sometimes also referred to as cold conductors, are metals. For example platinum-based temperature sensors (Pt100) are often used in high temperature applications such as in furnaces. Such sensors exhibit a good linearity but have only a comparably small temperature coefficient of about 3.9‰ per ° C. Other materials are semiconducting polycrystalline ceramics such as BaTiO3 which build up a depletion layer on grain boundaries. Although these materials have a higher temperature coefficient than many metals, their linearity is unsatisfactory.
NTC-resistors are, for example, pure semiconductor materials, the carrier density of which increases with the temperature which results in a reduced resistance at elevated temperatures. However, the resistance of the semiconductor materials obeys an exponential temperature dependence.
Another option for measuring the temperature is the use of a forwardly biased pn-junction, the resistance of which is temperature dependent. A pn-junction has a good linearity but only a limited temperature resolution in a range of about −2 mV per ° C. This is often too small to obtain a temperature resolution of about 5° C. since manufacturing variations may lead to deviations between individual temperature sensors which can be higher than 10 mV. To avoid parasitic heating of the temperature sensors caused by the current flow therethrough, only small currents of about 1 mA/mm2 should be supplied to the pn-junction. This reduces the temperature coefficient even further. Hence, an individual calibration is needed. Replacement or exchange of power devices can therefore only take place after careful pre-selection with respect to the temperature characteristics of their temperature sensors.